1. Field of the Invention
The present invention relates to an image measurement method, an image measurement device, an exposure apparatus, a substrate for image measurement, and a device manufacturing method.
2. Description of the Related Art
In order to evaluate the performance of a projection optical system of an exposure apparatus while the projection optical system is mounted on the main body of the exposure apparatus, a wafer coated with resist is generally used, and the wafer is exposed to a mask pattern. In this evaluation method, after the exposure, the resist is developed to form a resist pattern, and the resist pattern is measured with, for example, a scanning electron microscope (SEM). The evaluation method needs steps of resist application, development, and measurement, and hence, a single evaluation may take a long time, and need a large cost.
Therefore, a measurement method has been performed in which an image of a mask pattern or a measurement pattern is formed in the air at a position corresponding to a wafer surface, and the light intensity distribution of the formed image is directly measured with a measurement instrument, without the actual exposure (hereinafter, referred to as an aerial image measurement method). An example of this method may be a slit-scan method in which a slit having a width smaller than a wavelength of light from a light source is scanned, and light transmitted through the slit is measured with a photodetector, to measure the light intensity distribution having the size smaller than the wavelength of the light from the light source (refer to W. N. Partlo, C. H. Fields and W. G. Oldham, “Direct aerial image measurement as a method of testing high numerical aperture microlihographic lenses”, J. Vac. Sci. Technol. B, Vol. 11, pp. 2686-2691).
The slit-scan method uses, for example, a slit 540 which is formed at a light-shielding film 51 as shown in FIG. 10. FIG. 11 shows a schematic cross section of a measurement device using the slit-scan method, taken along line A0-B0 in FIG. 10. A line/space (hereinafter, referred to as L/S) pattern is illuminated, and its image is formed as to form an aerial image 40 having a periodic intensity distribution. A part of light of the formed aerial image 40 is transmitted through the slit 540. The light transmitted through the slit 540 is transmitted through a transparent substrate 52 which supports the light-shielding film 51, and then is emitted on a photodetector 53. The light emitted on the photodetector 53 is photoelectrically converted, and is output as a slit signal SS. A sensor 50, which includes the light-shielding film 51, the transparent substrate 52, and the photodetector 53, is scanned by a stage 60 in the x direction. A slit signal SS is monitored every scanning step. The signal obtained by slit-scanning and monitoring (hereinafter, referred to as a slit-scan signal) is used to measure the aerial image 40. The slit-scan signal is a signal in which a slit signal SS is modulated depending on a scanned position of the sensor 50.
Unfortunately, with the above slit-scan method, when the pitch of variation in the intensity distribution of the aerial image 40 becomes short, the modulation factor of the slit-scan signal may be significantly degraded if the longitudinal direction of the slit 540 is shifted from a line-extending direction of the L/S pattern in the aerial image 40. The modulation factor is expressed by (maximum value-minimum value)/(maximum value) of the light intensity. FIG. 12 shows the longitudinal direction of the slit 540, and the direction in which the aerial image of the L/S pattern is formed. The aerial image 40 is an image of the L/S pattern formed in parallel to the y direction. The light intensity distribution is modulated by a half pitch HP in the x direction. The longitudinal direction of the slit 540 formed at the light-shielding film 51 is shifted with respect to the direction (y direction) parallel to the L/S pattern of the aerial image 40, by an angle θ in a rotation direction in the x-y plane. Ideally, if θ is substantially zero, a slit-scan signal of a high modulation factor can be obtained. However, in fact, θ is not zero due to an alignment error or the like.
When the slit 540 is scanned in the x direction in this state, the modulation factor of the slit-scan signal may be degraded more than the modulation factor of the aerial image 40. Further, if the position of the slit 540 is shifted such that the slit 540 extends over the pitches of the L/S pattern of the aerial image 40, the light quantity of light emitted on the slit 540 would not be changed although the slit 540 is scanned. The modulation factor of the slit-scan signal becomes substantially zero, and hence, the measurement is no longer available. Assuming that an angle defined by the slit 540 and the L/S pattern is θc, the angle θc can be expressed as follows:θc=arc sin (2HP/SL)where SL is a length of a slit in the longitudinal direction, and HP is a half pitch of the variation in light intensity distribution of the aerial image 40. If the angle defined between the slit 540 and the L/S pattern is smaller than θc, the light quantity of light emitted on the slit 540 is modulated by slit-scanning.
FIG. 13 plots the function according to HP and SL. The vertical axis represents θc. As shown in FIG. 13, in a case where the slit length SL is about 50 μm, θc is about 14 mrad when HP is 200 nm. In contrast, when HP is 45 nm, θc becomes markedly small as about 2 mrad. That is, as HP decreases, θc decreases. To obtain a slit-scan signal with a high modulation factor having a small tolerance of the position shift, a high alignment accuracy allowing the position-shift angle θ to be at least smaller than θc is required.
In other words, when the light intensity distribution of light with a spatial variation pitch smaller than a wavelength of light from a light source is measured by the slit-scan method, if the longitudinal direction of a slit is shifted from a direction in which an one-dimensional-space light-intensity distribution does not vary, the modulation factor of the signal to be measured may be degraded.